1. Field of the Invention
The present invention relates to a method of decomposing a plastic waste by a chemical reactions, in which water in a supercritical or near supercritical region is used as a reaction medium, and thereby converting the plastic waste into oil, and further relates to an apparatus therefor.
2. Description of the Related Art
Hitherto, almost all of the plastic waste of various types have been dumped on a land reclamation site or have been burned and thus have not been effectively utilized as resources. Further, in the case of dumping the plastic waste on a reclaimed land, it is difficult to secure a land or ground to be reclaimed and the reclaimed ground is unstable. Further, in the case of burning the plastic waste, there have been caused the problems that because these wastes are high-calorie solids, an incinerator is badly damaged and that the plastic waste gives off a poisonous gas and a foul odor during incineration thereof.
In such a context, in recent years, attempts have been made to develop a method for recycling and reusing a plastic waste without pollution, and for effectively utilizing the recycled plastics as resources. As one of such methods, there has been proposed a method (namely, what is called a supercritical water method or process) of decomposing a plastic waste by a reaction (namely, a supercritical water reaction), which uses water in a supercritical region (namely, supercritical water) or water near the supercritical region (namely, near supercritical water) as a reaction medium, to thereby convert the plastic waste into oil and of collecting useful oil-like materials (refer to Japanese Patent Provisional Publication 56-501205 corresponding to WO81/00855 and Japanese Patent Provisional Publication Nos. 57-4225, 5-31000, 6-279762, and U.S. Pat. No. 4,113,446; all of these references are incorporated herein by reference).
Supercritical water is defined as water that is in conditions in which the temperature is critical temperature (namely, 374xc2x0 C.) of water or higher and the pressure is the critical pressure (namely, 22.1 MPa) or higher. In the case of the supercritical water method, the quantity of generated gas is low and the amount of a residue is small, in comparison with the case of utilizing thermolysis at ordinary pressure. Thus, a high oil conversion ratio, at which a plastic waste is converted into oil, can be realized. Incidentally, even if the pressure of water is not higher than the critical pressure, the water acting as a reaction medium behaves almost similarly as supercritical water does when the pressure thereof is close to the critical pressure (namely, the ratio of the pressure thereof to the critical pressure is 0.8 or more).
However, with the current state of the art, techniques for treating a plastic waste according to the supercritical water method have not been sufficiently clearly established yet. Especially, from the point of view of the speed-up of the reaction and the treatment (or disposal) of large amounts of plastic waste, the current techniques may not be effective and are not suitable for industrialization. Namely, in the case of the treatment of by the conventional supercritical water method, a batch treatment is performed (by a reactor of the autoclave type) at a temperature of 400xc2x0 C. or so, with the result that it takes long time, which is 30 minutes or more, to perform the decomposition of a plastic waste and convert the plastic waste into oil. Moreover, it is difficult to treat large amounts of plastic waste.
Furthermore, when the plastic waste contains chlorine-base (or chlorine-containing) plastic such as vinyl chloride resin, there are caused the problems that the corrosion of the reactor and piping is effected by hydrogen chloride produced by the decomposition of the chlorine-base plastic contained in the plastic waste during the decomposition of the plastic waste and the conversion thereof into oil. Namely, a supercritical water reaction is effected in high-temperature water. Further, hydrogen chloride is ionized and chlorine ions are dissociated therefrom. Thus, conditions for accelerating the corrosion are produced. Consequently, the conventional treatment of a plastic waste by the supercritical water method has encountered another problem that metallic materials are seriously corroded.
Therefore, in the case that a plastic waste containing chlorine-base plastic is decomposed and converted into oil by the supercritical water method, it is very difficult to select the metallic materials for the reactor and piping. Thus, heretofore, in the case of using the supercritical water method, with the existing state of the art, only a plastic waste containing no chlorine-base plastic has been converted into oil after the chlorine-base plastic is fractionated and separated from the plastic waste by a pretreatment. The present invention is accomplished in view of such a state of the conventional art.
Accordingly, a first object of the present invention is to provide a method and apparatus of treating a plastic waste, which is suitable for industrialization and can decompose a plastic waste and convert the plastic waste into oil at a high speed in a supercritical water process for treating a plastic waste and further can treat large amounts of plastic waste.
Further, a second object of the present invention is to provide a method and apparatus of treating a plastic waste, which can effectively suppress the corrosion of the apparatus due to hydrogen chloride generated during the decomposition of the plastic waste and the conversion thereof into oil.
To achieve the foregoing objects, in accordance with an aspect of the present invention, there is provided a method of decomposing a plastic waste by a reaction using water in a supercritical or near supercritical region as a reaction medium and converting the plastic waste into oil, which comprises the step of carrying out the reaction in a tubular type continuous reactor.
It is preferable that the residence time of the plastic waste in the reactor is 5 minutes or less when conducting the reaction according to this method of converting the plastic waste into oil.
Further, in accordance with another aspect of the present invention, there is provided a method of decomposing a plastic waste by a reaction using water in a supercritical or near supercritical region as a reaction medium and converting the plastic waste into oil, which comprises the steps of: heating a plastic waste to decompose part or all of chlorine-base plastic included in the plastic waste and separate and remove generated hydrogen chloride therefrom; and conducting the reaction by mixing the plastic waste, from which the hydrogen chloride is separated, with water.
In the case of this method of converting a plastic waste into oil, it is preferable that a tubular type continuous reactor is used as the reactor for conducting the reaction.
Further, in such a case of this method of converting a plastic waste into oil, it is preferable that the residence time of the plastic waste in the reactor is 5 minutes or less when conducting the reaction according to this method of converting the plastic waste into oil.
Moreover, in accordance with still another aspect of the present invention, there is provided an apparatus for decomposing a plastic waste by a reaction using water in a supercritical or near supercritical region as a reaction medium and for converting the plastic waste into oil, which comprises a tubular type continuous reactor as a reactor for conducting the reaction.
Furthermore, in accordance with yet another aspect of the present invention, there is provided an apparatus for decomposing a plastic waste by a reaction using water in a supercritical or near supercritical region as a reaction medium and for converting the plastic waste into oil, which comprises: a hydrogen-chloride decomposing/removing mechanism for heating a plastic waste to decompose part or all of chlorine-base plastic contained in the plastic waste and separate and remove generated hydrogen chloride therefrom; and a reactor for decomposing the plastic waste, from which the hydrogen chloride is separated, by conducting the reaction.
Additionally, in the case of this apparatus for converting a plastic waste into oil, it is preferable that a tubular type continuous reactor is used as the reactor for conducting the reaction.
In accordance with the present invention, the decomposition of a plastic waste and the conversion thereof into oil are performed by a supercritical water reaction. As described above, supercritical water is defined as water whose temperature is at the critical temperature of water or higher and whose pressure is at the critical pressure of water or higher. However, even if the pressure of this water is not higher than the critical pressure, the behavior of the water acting as a reaction medium results in its showing activity which is almost similarly as of supercritical water when the pressure thereof is close to the critical pressure (namely, the ratio of the pressure thereof to the critical pressure is 0.8 or more). Therefore, supercritical water reactions according to the present invention include those caused in the case that the pressure of supercritical water is operated in the vicinity of the critical pressure of water.
The aforementioned methods and apparatuses of the present invention use tubular type continuous reactors as the reactors for conducting supercritical water reactions. A tubular type continuous reactor is one adapted to perform the decomposition of a plastic waste therein and the conversion of the plastic waste into oil by permitting a mixture of water, whose condition is maintained in a supercritical or near supercritical region, and the plastic waste to continuously flow therethrough. As a more practical example of such a reactor, a multitubular type reactor constituted by tying a plurality of thick-walled reaction tubes in a bundle can be mentioned. The reaction tubes can be made from suitable metals such as stainless steel.
Further, the present invention, which relates to the aforesaid methods and apparatuses, aims at suppressing the corrosion of the apparatuses used in supercritical water reactions by heating a plastic waste to decompose part or all of chlorine-base plastic contained in the plastic waste and separate and remove generated hydrogen chloride, which is in a gaseous state, therefrom, before the step of mixing a plastic waste with water, hot water or supercritical water, for conducting supercritical water reactions. In this case, it is necessary for carrying out a supercritical reaction to mix the plastic waste with water or the like after separating and removing hydrogen chloride produced by the decomposition of chlorine-containing plastic. If a plastic waste is mixed with water or the like before separating and removing gaseous hydrogen chloride, the hydrogen chloride is ionized and chlorine ions are dissociated therefrom. This accelerates the corrosion of the apparatus. Thus, the objects of the present invention are not achieved.
However, the decomposition of a plastic waste and the conversion of the plastic waste into oil can be achieved in a short time which is 5 minutes or less, by using the tubular type continuous reactor as the reactor and by treating the plastic waste at temperatures which are a specific temperature or higher. Thereby, a large amount of a plastic waste can be continuously treated or processed.
Although the methods of the present invention are to perform the decomposition of a plastic waste and the conversion of the plastic waste into oil under supercritical conditions, the range of preferable reaction temperatures varies with the kind of plastic that is a main part of the plastic waste. To carry out a reaction in a short time, it is preferable to set the reaction temperature at a value that is 410xc2x0 C. or higher. For instance, in the case of the plastic waste mainly composed of a polystyrene resin or a methacrylic resin, the temperatures ranging from 410 to 600xc2x0 C. are preferable. Further, the temperatures ranging from 420 to 480xc2x0 C. are particularly preferable. Moreover, in the case of the plastic waste mainly composed of polyethylene, polypropylene, polyvinyl chloride, polyvinylidene chloride, ABS resin, polyvinyl alcohol, polycarbonate, polyester resin or polyamide, the temperatures ranging from 480 to 600xc2x0 C. are preferable. Further, the temperatures ranging from 490 to 550xc2x0 C. are particularly preferable.
The reaction pressure is set in such a manner as to be equal to or higher than the critical pressure of water, or to be close to the critical pressure thereof. In view of the speed-up of a reaction and the allowable operating temperature and the permissible working pressure of materials used in the reactor, further preferable pressures range from 18 to 50 MPa. Particularly preferable pressures range from 22.1 to 35 MPa.
In the case of the methods and apparatuses of the present invention, a plastic waste is mixed with water, hot water or supercritical water to thereby cause a supercritical water reaction. The plastic waste may be mixed with such supercritical water or the like while the plastic waste is heated and melted. Alternatively, the plastic waste, which is crushed into fragments, may be mixed with such supercritical water or the like.
Further, if a chlorine-containing plastic such as polyvinyl chloride and polyvinylidene chloride is included in a plastic waste, the corrosion of the apparatus occurs when conducting a supercritical water reaction, as above described. Therefore, if a plastic waste including such chloride-containing plastic is treated, it is preferable that the hydrogen-chloride decomposing/removing mechanism is provided in the apparatus, before performing the step of mixing the plastic waste with water, hot water or supercritical water, and that thus the plastic waste is heated and melted to thereby decompose part or all of chlorine-base plastic contained in the plastic waste and separate and remove generated gaseous hydrogen chloride therefrom.
Heating temperature, at which the plastic waste is heated for removing hydrogen chloride therefrom, is suitably set according to the composition and properties of the waste. Because the generation of decomposition gases other than hydrogen chloride should be suppressed, the preferable heating temperatures range from 200 to 400xc2x0 C. Further, particularly preferable heating temperatures range from 250 to 350xc2x0 C.
In the case of the methods and apparatuses of the present invention, a plastic waste can be efficiently decomposed and converted into oil in a short time by using a tubular type continuous reactor as a reactor. Thereby, a large quantity of a plastic waste can be continuously treated. Moreover, the methods and apparatuses of the present invention are made to be more effective by setting the reaction temperature at a value within a specific range according to the kind of a plastic waste which is a major constituent of the plastic waste. Namely, the present invention enables the speed-up of the reaction and the continuous treatment of large amounts of plastic waste. Moreover, a reduction in size of the reactor can be achieved. Thus, the industrialization of the treatment of a plastic waste by the supercritical water method can be attained.
Furthermore, in the case of the method and apparatus of the present invention, when the decomposition of a plastic waste containing chlorine-base plastic and the conversion of the plastic waste into oil are carried out as a result of performing the supercritical water process by conducting a supercritical water reaction after removing hydrogen chloride by performing the pretreatment of a plastic waste, the corrosion of the apparatus due to hydrogen chloride generated from the chlorine-base plastic can be suppressed effectively.
Additionally, oil-like materials, which contain almost no chlorine ingredient and can be effectively utilized as fuel oil, can be collected in good yield from a plastic waste containing chlorine-base plastic.